The energy from UVB is largely absorbed within the epidermis. It produces characteristic pyrimidine dimers and (6–4) photoproducts, as well as inducing oxidative damage to DNA. By contrast, the lower energy UVA penetrates well into the dermis. It induces reactive oxygen species and consequent oxidative DNA damage, particularly involving guanine. It has been demonstrated that some genes are especially prone to UV-induced damage; including the tumour suppressor gene p53. Inflammation has been shown to enhance experimental UV photocarcinogenesis. This suggests that the full UV spectrum incident on the skin, and the local inflammation associated with cellular damage in response to burning, all contribute to carcinogenesis.

UV exposure also induces immunosuppression. This is attributed to products from injured keratinocytes, in particular interleukin 10. Immunosuppression is both local and systemic, with antigen specific immune tolerance being induced as well as effector cell inhibition. Both UVA and UVB contribute to this immunosuppression.

With the appreciation of the role played by UVA, the Australian Standard governing sunscreens was upgraded in 2012. The changes established a standard for in vivo sun protection factor testing (measurement of protection against UVB), and a more stringent in vitro testing for UVA protection (termed broad spectrum protection). There is currently no requirement for testing for protection against immunosuppression; although a number of non-standardised in vivo and in vitro techniques have been described for deriving an immunoprotection factor.

On a population basis, Olsen and colleagues asserted that in 2008, as a result of sunscreen use, 14 190 Australians were prevented from developing an SCC, representing a prevention fraction (the proportion of those who would otherwise have developed the condition) of 9.3%, and 1730 Australians were prevented from developing a melanoma, a prevention fraction of 14%. They cited data from other studies for SCC, showing a prevention fraction of between 9.3 and 17%. Gordon and colleagues have claimed that, with regular sunscreen use, there was an accrued saving to Medicare for a population of 812 residents of Nambour over a 5-year study period of the order of US$88 000.

Active chemoprevention takes the sun protection debate further. Nicotinamide has been shown in a phase 3 trial to exert a protective effect with reduction in actinic keratoses, SCC and superficial BCC when taken orally in a dose of 500 mg twice daily. The authors reported a high level of safety and tolerability. Oral retinol, isotretinoin and acitretin also exert a protective effect but with a higher incidence of adverse effects. Some sunscreen and cosmetic manufacturers are incorporating nicotinamide in their products. While there are data to show that this may reduce UV-induced immunosuppression, in the absence of product-specific corroboration, this should be expanded into the commercial product setting with caution.

In summary, there are ample data to demonstrate the efficacy of sunscreens in skin cancer prevention. These should be combined with other sun protection behaviours, such as optimal timing of outdoor activities and appropriate choice of protective wear, including broad-brimmed hats, sunglasses and clothing. Chemoprevention with safe agents such as nicotinamide should be considered; however, its role as a primary intervention has not been established. Agents such as oral retinoid medications should be reserved for identified high risk populations, as a secondary intervention.

Patients at higher risk of developing skin cancer include those with a past history of skin cancer, patients who are immunosuppressed or immunocompromised, people who have a family history of melanoma or atypical moles, and patients taking medications associated with phototoxicity. These patients should remain a focus for active sun protection counselling and regular skin examinations.

Dr Andrew Miller is a dermatologist who has worked in private practice for over 20 years, including as a visiting medical officer at both Calvary Hospital and the Canberra Hospital, where he is department head. He is a member of the Australian Medical Association Australian Capital Territory board and the Australian Medical Association Federal Council.

To find a doctor, or a job, to use GP Desktop and Doctors Health, book and track your CPD, and buy textbooks and guidelines, visit doctorportal.